The present invention relates to subscriber line interface circuits, and more particularly, to a subscriber line interface circuit for controlling AC and DC output impedance.
Telephone communication systems use a subscriber line interface circuit (SLIC) at the central office to interface each phone user to the telephone network. For example, a residential phone site is connected to the central office by a communication loop comprising two transmission lines, commonly referred to as TIP and RING, nominally energized to 48 volts DC with an AC signal modulated for the voice data. The TIP and RING lines come in a twisted-pair configuration from the residential phone and are eventually coupled to one SLIC at the central office. The other side of the SLIC board provides a TX signal and receives an RX signal from other SLIC boards. Thus, a complete telephone communication originates from one party, travels over first TIP and RING lines, and is processed through a first SLIC and out as a TX signal to the RX input of a second SLIC for transmission over second TIP and RING lines to the receiving party.
The SLIC provides a predetermined AC and DC termination impedance for the TIP and RING lines at the central office. In a DC sense, the TIP and RING lines are matched each side to 200 ohms.+-.10% looking into the central office. The DC termination limits DC current flowing through TIP and RING. In an AC sense, the TIP and RING lines each see 100-800 ohms .+-.0.1% common mode and 300-450 ohms .+-.0.1% differential depending on the telephone system. A proper AC termination is needed to suppress common mode voltage spikes, e.g., from inductive pick-up when TIP and RING are placed near 110 VAC residential power wires.
A common design approach for the SLIC, as described in U.S. Pat. No. 4,476,350, is to use operational amplifiers (op amps) because of their bi-directional properties. One problem with known SLIC designs is that the op amps are DC-wired to TIP and RING. That is, the 48 VDC signal level on TIP and RING follows DC paths through resistors and appears at the inputs of the op amps. Hence, the op amps must operate with at least 48 VDC power supply potential.
Unfortunately, op amps running off a 48 VDC power supply tend to consume more power than ones operating at a lower power supply potential. With the large number of telephone users connected to the central office, each requiring a dedicated SLIC, power dissipation becomes significant. Although the SLIC may be idle the vast majority of the time when the phone is on-hook, it still must be biased ready to transmit large amounts of power, on the order of 400 milliwatts, when the call comes through and the phone is pulled off-hook for voice communications. At 48 VDC operating potential, each SLIC may dissipate upwards of 90 milliwatts between calls. Moreover, with the introduction of tele-metering and caller-ID, SLICs must pass AC signals while the phone is on-hook. Thus, power requirements are increasing even during times the phone is on-hook. Any reduction in power consumption by operating one or more op amps in the SLIC at a lower power supply potential translates to a considerable power savings.
Hence, a need exists for a SLIC operating one or more of its op amps at a reduced power supply potential to reduce overall power consumption.